Burner systems, which are operated in accordance with the concept of lean premix combustion, have low pollutant emissions but also a distinctly limited stability range. In addition to flashback into the mixing zone and to lift-off or quenching of the premix flame, thermoacoustic vibrations lead to distinct limitations in the operating behavior.
The operating behavior of premix burners can be improved by means of a staged fuel supply (see, for example, printed publications EP0797051 A2 or EP1205653 B1 or EP1292795 B1 or EP1344002 B1 or W02007/082608, which are incorporated by reference).
The fuel staging via a central device installed in the burner is shown in FIG. 1 by way of example for a double-cone burner or EV burner according to printed publication EP1292795 B1. FIG. 1 shows a premix burner 10, which has a double cone 12, into which leads a central fuel supply 11. The centrally injected fuel constitutes a first fuel stage 13. A second fuel stage 14 is created by additional fuel being injected into the air feed slots of the double cone 12.
By adjustment of the fuel proportion via the central fuel injection 11, 13 (fuel stage no.1) in relation to the fuel injection in the burner air slot 12, 14 (fuel stage no.2), the operating range of the burner 10 can be broadened with regard to quenching or lift-off of the premix flame and also with regard to flashback. The fuel staging also offers the possibility of optimizing the operating range of the burner 10 with regard to thermoacoustic vibrations and pollutant emissions.
Shown in FIG. 2 is a further premix burner 20 according to printed publication EP 0797051 A2 with a further design of the fuel staging. In the depicted design, the burner 20 has a double cone 16 to which is connected a mixer tube 17 towards the combustion chamber 15. An external fuel feed 18 is arranged at the end of the mixer tube 17. The fuel which is supplied there forms a fuel stage 19 (fuel stage no.1). The fuel which is supplied via the burner air slots of the double cone 16 forms a fuel stage 21 (fuel stage no.2). This depicted burner system can also be optimized by fuel staging with regard to thermoacoustic vibrations and pollutant emissions. In contrast to the burner system shown in FIG. 1 with central injection, premix burners with external fuel injection have an appreciably broadened operating range with regard to quenching or lift-off of the premix flame.
The parameters for the fuel staging—if the burners are part of a gas turbine—are determined by tuning runs during commissioning of the gas turbine. The parameters of the fuel staging can be predetermined in this case as a function of the output of the gas turbine, the ambient conditions, such as ambient temperature and air humidity, and the fuel composition. The parameters must also be predetermined with an adequate reserve so that even with transient operating states, such as loading or unloading of the gas turbine or changes to the gas composition, a reliable operation is ensured. Since these transient operating states in gas turbines very seldom occur, the premix burner cannot be operated to the greatest possible extent with minimum pollutant emissions.
A method for operating a premix burner with staged fuel injection is now known from printed publication EP1205653 B1, which has at least one first stage and at least one second stage, arranged downstream of the first stage, for introducing fuel into a combustion air flow, wherein pulsations of a combustion initiated by the burner and/or emission values of the combustion are detected, and the fuel feed to the first stage and second stage is controlled as a function of the detected pulsations and/or emission values. During steady-state operation, the fuel feed in this case is controlled in such a way that the operating point lies (permanently) below the maximum of the pulsations. During transient operation, on the other hand, the fuel feed is controlled in such a way that the operating point can lie (permanently) above the maximum of the pulsations. As a result, the reliability of the gas turbine may be limited during transient operation.
A method for controlling the ratio of the fuel flows in a first fuel supply line and in a second fuel supply line to a combustion device is known from printed publication W02007/082608, in which it is first of all determined whether the value of a first parameter—which shifts the operating point of the device towards a first undesirable operating range—has exceeded a predetermined limit value. If this limit value has been exceeded, the ratio of the fuel flows is changed so that the value of the first parameter falls short of the limit value. If this limit value has not been exceeded, it is determined whether the value of a second parameter—which shifts the operating point of the device towards a second undesirable operating range—has exceeded a predetermined limit value. If this limit value has been exceeded, the ratio of the fuel flows is changed so that the value of the second parameter falls short of the limit value. If this limit value has not been exceeded, the aforesaid steps are repeated in order to hold the values of the first and second parameters below the respective limit value. This type of controlling is comparatively costly and complicated.